1. Field of the Invention
The present invention relates to a positive electrode of a rechargeable lithium ion battery using a non-aqueous electrolytic solution and a rechargeable lithium ion battery and a battery module having the positive electrode, and more particularly, to an improvement in structure of a positive electrode.
2. Background Art
For the purpose of further improvement in energy efficiency of vehicles, there is a need for development of a plug-in hybrid electric vehicle (hereinafter, abbreviated as “PHEV”). Since a PHEV uses energy stored with a household power source to travel, the battery used in the PHEV requires a high capacity necessary for an electric vehicle having a long cruising range and high output power for a short time necessary for a hybrid vehicle.
In this way, an increase in capacity and an increase in output power are important as battery characteristics necessary for the PHEV. Accordingly, since a rechargeable lithium ion battery for a PHEV is a large-size high-capacity battery, the ensuring of safety is important. In an in-vehicle large-size high-capacity rechargeable lithium ion battery, there is a need for improvement in volume energy density and weight energy density for the purpose of a decrease in size and weight of a battery. Since the large-size high-capacity rechargeable lithium ion battery stores great energy, a positive-electrode active material having high thermal stability and high safety is required.
A positive-electrode active material (LiMPO4 where M is a transition metal including at least one of Fe and Mn and which is hereinafter referred to as an “olivine positive-electrode material”) having an olivine structure including Fe or Mn as a transition metal has attracted attention as a positive-electrode material satisfying the above-mentioned requirements. In the olivine positive-electrode material, since the bond of oxygen and phosphorous in the crystal structure is strong and oxygen is not easily discharged from the crystal structure at the time of overcharging, it has high safety. However, it is reported that the olivine positive-electrode material has low electron conductivity and a low diffusion coefficient of lithium ions into the positive-electrode material.
In the olivine positive-electrode material, the diffusion properties of lithium ions are improved by enhancing the specific surface area of the material for the purpose of practical use, and conductivity is given thereto by coating the material with carbon (carbon coating). When a material is coated with carbon, it is possible to give conductivity to the material and to suppress the crystal growth, thereby contributing to an increase in specific surface area due to a particle diameter reduction of reducing primary particles to a sub-micrometer size.
From the viewpoint of improvement in volume energy density, the olivine positive-electrode material has the following problems. For example, since the true density of olivine Fe is 3.6 g/cc (g/cm3), the volume of the olivine positive-electrode material increases to acquire the same volume energy density as the positive-electrode material employing a layered LiNiMnCoO2 system with a true density of 5.1 g/cc. Accordingly, the olivine positive-electrode material is a material of which the volume density it is difficult to increase. In addition, the density of the olivine positive-electrode material further decreases when it is coated with carbon. Since the olivine positive-electrode material has a large specific surface area as described above, the amount of binder per unit surface area necessary for forming an electrode increases. However, in order to guarantee the battery capacity, it is preferable to reduce the amount of binder in the electrode composition.
In general, in order to enhance the volume energy density in a high-capacity battery, it is necessary to increase the content of a positive-electrode active material in the positive electrode and to increase the thickness of a mixture layer including a positive-electrode active material, a conducting agent, and a binder. In manufacturing an olivine positive electrode, a slurry in which an olivine positive-electrode material, a conducting agent, and a binder are dispersed in a solvent is applied onto an aluminum collector and the resultant is dried to obtain a positive electrode. In an electrode with a thick mixture layer, a phenomenon in which a binder resin migrates to the surface layer with the evaporation of the solvent in the drying step markedly occurs. Accordingly, the amount of binder in the interface between the aluminum collector and the mixture layer decreases. When the amount of binder decreases, the mixture layer is detached from the interface by the consolidation due to an electrode pressing process or a rolling process. In this way, the consolidation of an electrode for improvement in volume energy density is indispensable for the olivine positive electrode, but it is necessary to suppress the detachment of the mixture layer from the interface between the aluminum collector and the mixture layer.
JP-A-2010-212167 discloses a method of forming a carbon coating layer with a surface roughness of 0.5 to 1.0 μm on a collector and forming a mixture layer thereon so as to suppress detachment of the mixture layer from the interface between the collector and the mixture layer. It also discloses that the load characteristics can be improved by employing this configuration.